1. Field of the Invention
The present invention relates to an antenna used in, for example, communication equipment such as mobile telephones and a high frequency component mounted on such communication equipment, more specifically, an antenna using variable capacitance element having excellent characteristics in terms of power handling capability, low distortion and low loss in which the electrical length of the antenna can be varied by changing the capacitance using a variable capacitance capacitor, and a wireless communication apparatus using the antenna.
The invention relates to an antenna used for wireless communication in a microwave band, a millimeter-wave band or the like in which a variable matching circuit having a variable capacitance capacitor is connected between a feeding terminal of an antenna element and a feeding power source, and an operating frequency of the antenna can be varied by changing the capacitance of the variable capacitance capacitor, and a wireless communication apparatus using the same. In particular, the invention relates to an antenna having excellent characteristics in terms of power handling capability, low distortion and low loss, and a wireless communication apparatus using the same.
2. Description of the Related Art
An antenna is one of the principal components for communication equipment, and of these components, the antenna is a large component and is desired to be smaller with the promotion of the miniaturization of the equipment. The size of the antenna has a close relationship with a bandwidth and has a size necessary to ensure the bandwidth that is required by the system of the communication equipment.
On the other hand, when the operating frequency of the antenna is variable, the antenna only has to have a size necessary to ensure the bandwidth at that point of time, and therefore it is not necessary that the size of the antenna is large so as to be ready for the bands that are not used. Therefore, the size of the antenna can be reduced significantly.
Then, in order to make the operating frequency of the antenna variable, it has been proposed to connect a variable capacitance diode as a variable capacitor to the antenna or connect a resonance frequency adjusting circuit including a variable capacitance diode as a variable capacitance element to the antenna so as to change the operating frequency, or switch the capacitance that is to be connected to an antenna element with a switch so as to change the operating frequency (e.g., see Japanese Examined Patent Publication JP-B2 3307248 and Japanese Unexamined Patent Publication JP-A 2002-232232).
However, the variable capacitance diode has the problem derived from its material characteristics that the power handling capability is low, and the distortion characteristics due to the nonlinearity of the capacitance are large, so that the variable capacitance diode only can be used in receivers and receiving circuits having a small operating power. In other words, the variable capacitance diode cannot be used in transmitters and transmitting circuits having a large operating power. Furthermore, there is the problem that the loss at a high frequency is large.
In the antenna using a resonance frequency adjusting circuit including a variable capacitance diode that has been proposed in JP-B2 3307248, the variable capacitance diode has a capacitance variation at a high frequency voltage. Therefore, when the antenna has a high frequency voltage, the distortion characteristics such as waveform distortion or intermodulation distortion become large. In order to improve the distortion characteristics, it is necessary to decrease the high frequency electric field strength of the variable capacitance diode so as to reduce the capacitance variation due to the high frequency voltage. For this purpose, it is effective to increase the thickness of a capacitance layer made of a p-n junction layer in the variable capacitance diode. However, when the thickness of the capacitance layer is increased, the direct electric field strength is reduced, and the capacitance change ratio is decreased, and thus the control width of the frequency of the frequency variable antenna is decreased.
Furthermore, since with high frequency signals, current easily flows through the variable capacitance diode, during a period in which the variable capacitance diode is used in a resonance frequency adjusting circuit, which is also a high frequency circuit, heat is generated due to the loss resistance in the variable capacitance diode, and the variable capacitance diode is damaged. Thus, the power handling capability of the frequency variable antenna with respect to high frequency signals is low. To deal with this problem of power handling capability, it is effective to increase the thickness of the capacitance layer so as to reduce the amount of heat generated per unit volume. However, when the thickness of the capacitance layer is increased, the direct electric field strength is decreased, and the capacitance change ratio is decreased, and thus the control width of the frequency of the antenna is decreased.
In addition, when the variable capacitance diode is used, a bias signal supplied thereto is supplied to the variable capacitance diode from a bias terminal via a bias supply circuit. Therefore, an independent bias supply circuit constituted by a choke coil or the like that is separated from the variable capacitance diode is necessary. Therefore, it is necessary to design a bias supply circuit, and time and efforts are required for adjustment thereof. Moreover, since a circuit employing the variable capacitance diode and the bias supply circuit are constituted as separate components, the size as a whole is increased.
Furthermore, since the variable capacitance diode has a polarity with respect to an applied voltage, it is necessary to pay attention to the polarity not only at the time of design, but also at the time of mounting, and it takes time and efforts for this.
In the configuration in which the operating frequency is changed by switching a capacitor connected to an antenna element with a switch as disclosed in JP-A 2002-232232, the operating frequency that can be realized cannot be continuously variable and is limited to two different frequencies.
Furthermore, there is a demand for achieving higher performance and reducing the size of wireless communication apparatuses. Such a demand is strong especially for mobile telephones. In particular, there is an increasingly strong demand for further reduction of the size for the antenna, which is a component thereof, because the antenna is a leading component of the wireless communication apparatus but has a large size. There is a close relationship between the size of an antenna element constituting an antenna and the bandwidth, and in order to ensure the operating frequency and the bandwidth necessary for transmission of signals that is required by the communication system, the corresponding size is necessary.
On the other hand, it is possible to adjust the communication frequency to the necessary frequency as appropriate by making the operating frequency of the antenna variable. Therefore, even an antenna having a narrow bandwidth can be used. As a result, a compact antenna element in which the bandwidth is reduced can be used. Then, in order to make the operating frequency of the antenna variable, it has conventionally been proposed to connect a variable matching circuit having a variable capacitance element between an antenna element and a load, and to use a variable capacitance diode such as varactor diode as a variable capacitance element (e.g., see Japanese Unexamined Patent Publication JP-A 9-307331 (1997)). Furthermore, instead of the variable capacitance diode, a variable matching circuit combined with a variable capacitance capacitor also has been proposed (e.g., see Japanese Unexamined Patent Publication JP-A 11-111566 (1999)).
However, the antenna connected to the variable matching circuit having a variable capacitance diode as proposed in JP-A 9-307331 has the problem that the power handling capability is low, and the distortion characteristics due to the nonlinearity of the capacitance are large, so that the variable capacitance diode only can be used in receivers and receiving circuits having a small operating power. In other words, the variable capacitance diode cannot be used in transmitters or transmitting circuits having a large operating power. Furthermore, there is the problem that the loss at a high frequency is large.
In the variable matching circuit employing a variable capacitance capacitor as proposed in JP-A 11-111566, the variable capacitance capacitor has a capacitance variation also due to a high frequency voltage, and therefore when the variable matching circuit has a high frequency voltage, the distortion characteristics such as waveform distortion and intermodulation distortion become large. In order to improve the distortion characteristics, it is necessary to decrease the high frequency electric field strength of the variable capacitance capacitor so as to reduce the capacitance variation due to the high frequency voltage. For this purpose, it is effective to increase the thickness of a dielectric layer. However, when the thickness of the dielectric layer is increased, the direct electric field strength is reduced, and the capacitance change ratio is decreased, and as a result, in the variable matching circuit of the antenna, the control width of the operating frequency of the antenna is decreased.
Furthermore, since with high frequency signals, current easily flows through the variable capacitance capacitor, during a period in which the variable capacitance capacitor is used in a variable matching circuit, which is also a high frequency circuit, heat is generated due to the loss resistance in the variable capacitance capacitor, and the variable capacitance capacitor is damaged. Thus, the power handling capability of the variable matching circuit of the antenna with respect to high frequency signals is low. To deal with this problem of power handling capability, it is effective to increase the thickness of the dielectric layer so as to reduce the amount of heat generated per unit volume. However, when the thickness of the dielectric layer is increased, the direct electric field strength is decreased, and the capacitance change ratio is decreased, and as a result, in the variable matching circuit of the antenna, the control width of the operating frequency of antenna is decreased.
In addition, as shown in an equivalent circuit diagram of an example of a variable matching circuit in a conventional antenna in FIG. 14, a bias signal is supplied to a variable capacitance diode 201 from a bias terminal V via a bias supply circuit G. Thus, the independent bias supply circuit G that is constituted by a choke coil L2 is necessary in the variable matching circuit. Therefore, it is necessary to design the bias supply circuit G separately from the variable matching circuit, and time and efforts are required for adjustment thereof. Moreover, since the variable matching circuit and the bias supply circuit G are constituted as separate components, the size of the circuit as a whole is increased. The same applies to the case where the variable capacitance diode is replaced by a variable capacitance capacitor.
Furthermore, in the same manner as above, since the variable capacitance diode 201 has a polarity with respect to an applied voltage, it is necessary to pay attention to the polarity not only at the time of design, but also at the time of mounting, and it takes time and efforts for this.